1. Field of the Invention
The invention pertains to the preparation of an aryl mono-, di- and/or polyurethane by reacting a primary aromatic mono-, di- and/or polyamine with an O-alkyl-carbamate in the presence of an alcohol. The reaction is preferably carried out in the presence of urea.
2. Description of the Prior Art
On an industrial scale, N-aryl urethanes are normally produced by the reaction of alcohols with isocyanates or by the reaction of amines with chlorocarbonates. The isocyanates and chlorocarbonates used in these reactions are obtained by phosgenation of the corresponding amines or the corresponding alcohols. Houben-Weyl, Methods of Organic Chemistry, Vol. 8, pages 137, 120 and 101, (Georg Thieme Publishers, Stuttgart, 1952). These processes are very expensive and phosgene must be used with care because of its potential danger to man and the environment.
N-aryl urethanes are used as intermediates and end products. For instance, German Published Application 26 35 490 and U.S. Pat. No. 3,919,278 disclose the use of N-substituted urethanes for the manufacture of isocyanates. Because of their utility, many attempts have been made to develop better methods for preparing N-substituted urethanes. These methods and their shortcomings will be discussed.
German Published Application 21 60 111 describes a process for the manufacture of N-substituted urethanes by reacting an organic carbonate with a primary or secondary amine in the presence of a Lewis acid. There are several problems with this process. The conversion rates are rather low and the reaction times are long. Furthermore, N-alkylarylamines are always produced as by-products.
U.S. Pat. No. 2,834,799 describes a process for making carbamic and carbonic esters by the reaction of urea with alcohols in the presence of boron trifluoride. The problem with this method is that the boron trifluoride is required in equimolar quantities so that at least one molecule of boron trifluoride is used per molecule of produced carbamic ester and at least two molecules of boron trifluoride are consumed per molecule of carbonic ester. This process is not only expensive, but it causes problems in the environment because the boron trifluoride is produced in the form of the H.sub.3 N.BF.sub.3 adduct.
R. A. Franz et al. Journal of Organic Chemistry, Vol. 28, page 585 (1963) describe a process for making methyl-N-phenyl urethane from carbon monoxide, sulfur, aniline, and methanol. Very low yields are produced by this method; the yield does not exceed 25 percent even when there is a long reaction period.
U.S. Pat. No. 2,409,712 describes a process for making N-alkyl and N-aryl urethanes by the reaction of monoamines with urea (either N,N'-dialkyl- or N,N'-diarylurea is used) and alcohols at temperatures of 150.degree. C. to 350.degree. C. under increased pressure. It should be noted that this patent only describes the manufacture of N-alkylmonourethanes and does not mention the manufacture of N,N'-disubstituted diurethanes and polyurethanes. U.S. Pat. No. 2,677,698 also describes a process for the manufacture of N-substituted monourethanes. In this process, the urea is initially converted into the corresponding N,N'-disubstituted urea with monoamines, is then cleaned, and subsequently is reacted with an alcohol. The processes described are expensive and the yields are very low. Attempts to improve the yield by improving the methods of preparing and purifying the N,N'-disubstituted ureas have been unsuccessful.
Other processes have not been successful in eliminating the problems described thus far. U.S. Pat. No. 2,806,051 describes a process whereby N-substituted urethanes are produced by reacting aniline with urea and alcohol at a mole ratio of 1.0:1.2:2.0 at temperatures below 200.degree. C., preferably of 120.degree. C. to 160.degree. C. Even in the preferably used temperature range, this process produces only small yields of N-substituted urethanes if the reaction time is limited to a period which is practical in an industrial setting. In view of the problems with this process, it is not surprising that U.S. Pat. No. 3,076,007, which describes the manufacture of N-alkyl and N-cycloalkyl urethanes, does not incorporate the above-referenced methods in its process. It does, however, describe the reaction of phosgene with alcohols to form chloroalkylformates, and it describes their subsequent reaction with amines to form urethanes. It also discloses the reaction of amines with ethylene carbonate to form urethanes. German Published Application 27 16 540 describes a more recent variation of this process wherein aromatic urethanes are prepared by reacting dialkyl carbonates with N-ethyl amines.
It is also known that ethyl carbamates do not react with amines in boiling dioxane [D. G. Crosby and C. Niemann, Journal of the American Chemical Society, Vol. 76, page 4458 (1954)], and that the reaction of N-alkyl urethanes with alcoholic ammonia solutions at temperatures of 160.degree. C. to 180.degree. C. result in an alkali solution from which aminohydrochloride, urea, alkylurea and alkyl urethane can be isolated by means of hydrochloric acid after neutralization [M. Brander, Rec. trav. Chim., Vol. 37, pages 88-91 (1917)]. The referenced publications do not contain any disclosure concerning the reaction of aromatic primary amines with carbamates although it is known that the heating of ethyl carbamate with aniline at 160.degree. C. in a bomb tube will produce diphenylurea. See Annalen, Vol. 147, page 163 (1868).
U.S. Pat. No. 2,409,712, discloses that the reaction of aliphatic monoamines, urea and alcohol will produce alkyl urethanes. However, only small yields result even though excess urea is used. Since somewhat higher yields are achieved with less urea and at lower temperatures according to U.S. Pat. No. 2,806,051, one has to assume that higher mole ratios of urea to amines are disadvantageous. Diphenylurea and O-alkyl carbamate were determined as by-products of the synthesis of phenylurethane. The O-alkyl carbamate was isolated by means of distillation in addition to unreacted aniline. The formation of O-alkyl carbamate from urea and alcohol was therefore considered as an interferring secondary reaction. Since even the manuacture of N-monoalkylsubsituted urethanes from alkylamines, urea, and alcohols succeeds with moderate yields only, and since carbamates are produced as by-products, it is not surprising that the prior art does not teach the preparation of aryl mono-, di- and/or polyurethanes from arylamines and O-alkyl carbamates.
Because of the problems identified thus far, other methods of producing N-arylurethanes have been tried. Some have suggested that N-arylurethanes can be prepared by reacting nitroaromatics with carbon monoxide, and alcohols in the presence of catalysts. German Published Application 15 68 044 (U.S. Pat. No. 3,467,694) teaches that urethanes may be prepared by the reaction of organic nitro compounds, carbon monoxide, and hydroxyl-containing compounds in the presence of a catalysts consisting of a noble metal and a Lewis acid under essentially anhydrous conditions in the absence of hydrogen under increased pressure and at temperatures above 150.degree. C. German Published Application 23 43 826 (U.S. Pat. No. 3,895,054) teaches that urethanes can be prepared from hydroxyl group-containing compounds, carbon monoxide, and nitro-, nitroso-, azo- and azoxy group-containing compounds in the presence of sulfur, selenium, a sulfur and/or selenium compound and at least one base and/or water. German Published Application 26 23 694 (U.S. Pat. No. 4,080,365) describes the preparation of aromatic urethanes from the above-referenced starting compounds in the presence of selenium-containing catalyst systems as well as special aromatic amino and urea compounds. However, the use of these processes involve serious drawbacks. The toxic carbon monoxide and catalysts which are toxic or form toxic compounds during the reaction, such as hydrogen selenide and hydrogen sulfide, or catalysts which are very expensive and are difficult to recycle such as palladium, require great technical expenditure and costly safety measures.
None of the references cited discloses the preparation of aryl mono, di and/or polyurethane by reacting an aromatic amine with an O-alkyl carbamate in the presence of an alcohol at temperatures greater than 160.degree. C. Moreover, the processes described all involve several disadvantages. It is surprising that aryl mono, di and/or polyurethanes can be produced in one process stage with good yields by reacting carbamates with primary aromatic amines in the presence of an alcohol at temperatures greater than 160.degree. C. Prior teachings indicate that corresponding diureas are obtained from diamines and carbamates; for example, hexamethylenediurea is obtained from hexamethylenediamine and carbamates. Prior art also teaches that, although urea and alcohol may react to produce urethanes, they continue to react to form N,N'-disubstituted ureas in the presence of amines. See Houben-Weyl, Methods of Organic Chemistry, Vol. 8, pages 152, 140, and 161, (Georg Thieme Publishers, Stuttgart, 1952). These side reactions decrease the yield of the desired product.
Furthermore, German Patent 896 412 indicates that high molecular, spinnable condensation products may be produced from the diamides of carbonic acid such as urea and diamines. This result is likely to occur if the amino groups of the diamines are separated by a chain of more than three atoms. U.S. Pat. Nos. 2,181,663 and 2,568,885, for instance, disclose that high molecular polyureas with molecular weights of 8000 to 10,000 and greater, may be produced when diurethanes are condensed with diamines at temperatures of approximately 150.degree. C. to 300.degree. C. Moreover, mono- and polyurethanes can be split thermally into isocyanates, alcohols and possibly olefins, carbon dioxide, urea and carbodiimide, and these products can be split into products such as biurets, allophanates, isocyanurates, polycarbodiimides, and others. See The Journal of the American Chemical Society, Vol. 80, page 5495 (1958) and Vol. 48, page 1946 (1956).
In view of the problems disclosed in the prior art, it was surprising that our process, which involved very similar reaction conditions, would result in mono, di- and/or polyurethane with very good yields. It was particularly surprising because when diurethanes were prepared from the products mentioned in the previous paragraph according to the reaction conditions of our invention, good yields did not result.